Peter V. Pikhitsa

A novel hybrid carbon material

Both fullerenes and single-walled carbon nanotubes (SWNTs) exhibit many advantageous properties. Despite the similarities between these two forms of carbon, there have been very few attempts to physically merge them. We have discovered a novel hybrid material that combines fullerenes and SWNTs into a single structure in which the fullerenes are covalently bonded to the outer surface of the SWNTs. These fullerene-functionalized SWNTs, which we have termed NanoBuds, were selectively synthesized in two different one-step continuous methods, during which fullerenes were formed on iron-catalyst particles together with SWNTs during CO disproportionation. The field-emission characteristics of NanoBuds suggest that they may possess advantageous properties compared with single-walled nanotubes or fullerenes alone, or in their non-bonded configurations.

Parallel patterning of nanoparticles via electrodynamic focusing of charged aerosols

The development of nanodevices that exploit the unique properties of nanoparticles1,2 will require high-speed methods for patterning surfaces with nanoparticles over large areas and with high resolution3,4,5,6. Moreover, the technique will need to work with both conducting and non-conducting surfaces. Here we report an ion-induced parallel-focusing approach that satisfies all requirements. Charged monodisperse aerosol nanoparticles are deposited onto a surface patterned with a photoresist while ions of the same polarity are introduced into the deposition chamber in the presence of an applied electric field. The ions accumulate on the photoresist, modifying the applied field to produce nanoscopic electrostatic lenses that focus the nanoparticles onto the exposed parts of the surface. We have demonstrated that the technique could produce high-resolution patterns at high speed on both conducting (p-type silicon) and non-conducting (silica) surfaces. Moreover, the feature sizes in the nanoparticle patterns were significantly smaller than those in the original photoresist pattern.

Trapped charge-driven degradation of perovskite solar cells

Perovskite solar cells have shown unprecedent performance increase up to 22% efficiency. However, their photovoltaic performance has shown fast deterioration under light illumination in the presence of humid air even with encapulation. The stability of perovskite materials has been unsolved and its mechanism has been elusive. Here we uncover a mechanism for irreversible degradation of perovskite materials in which trapped charges, regardless of the polarity, play a decisive role. An experimental setup using different polarity ions revealed that the moisture-induced irreversible dissociation of perovskite materials is triggered by charges trapped along grain boundaries. We also identified the synergetic effect of oxygen on the process of moisture-induced degradation. The deprotonation of organic cations by trapped charge-induced local electric field would be attributed to the initiation of irreversible decomposition.

Three-Dimensional Assembly of Nanoparticles from Charged Aerosols

The capability of assembling nanoparticles into a desired ordered pattern is a key to realize novel devices which are based not only on the unique properties of nanoparticles but also on the arrangements of nanoparticles. While two-dimensional arrays of nanoparticles have been successfully demonstrated by various techniques, a controlled way of building ordered arrays of three-dimensional (3D) nanoparticle structures remains challenging. We report that a variety of 3D nanoparticle structures can be formed in a controlled way based on the ion-induced focusing, electrical scaffold, and antenna effects from charged aerosols. Particle trajectory calculations successfully predict the whole process of 3D assembly. New surface enhanced Raman scattering substrates based on our 3D assembly were constructed as an example showing the viability of the present approach. This report extends the current capability of positioning nanoparticles on surface to another spatial dimension, which can serve as the foundation of future optical, magnetic, and electronic devices taking the advantage of multidimensions.

Hotspot‐Engineered 3D Multipetal Flower Assemblies for Surface‐Enhanced Raman Spectroscopy

Novel 3D metallic structures composed of multipetal flowers consisting of nanoparticles are presented. The control of surface plasmon hotspots is demonstrated in terms of location and intensity as a function of petal number for uniform and reproducible surfaceenhanced Raman spectroscopy (SERS) with high field enhancement.

Electron field emission from nanocarbons: A two-process model

We show that the conventional consideration of the electron-field emission from nanocarbons cannot explain the experimentally observed results. We suggest a mechanism of the field emission occurrence from nanocarbons that can solve the existing puzzles. This mechanism implies two successive processes: (1) Tunneling through the low-energy barrier from the metallic region into the semiconducting region under the external macroscopic electric field and (2) tunneling through the high-energy barrier from the semiconducting region into a vacuum under the Coulomb field of an additional electron appearing in the first process.

Room temperature CO and H2 sensing with carbon nanoparticles

We report on a shell-shaped carbon nanoparticle (SCNP)-based gas sensor that reversibly detects reducing gas molecules such as CO and H(2) at room temperature both in air and inert atmosphere. Crystalline SCNPs were synthesized by laser-assisted reactions in pure acetylene gas flow, chemically treated to obtain well-dispersed SCNPs and then patterned on a substrate by the ion-induced focusing method. Our chemically functionalized SCNP-based gas sensor works for low concentrations of CO and H(2) at room temperature even without Pd or Pt catalysts commonly used for splitting H(2) molecules into reactive H atoms, while metal oxide gas sensors and bare carbon-nanotube-based gas sensors for sensing CO and H(2) molecules can operate only at elevated temperatures. A pristine SCNP-based gas sensor was also examined to prove the role of functional groups formed on the surface of functionalized SCNPs. A pristine SCNP gas sensor showed no response to reducing gases at room temperature but a significant response at elevated temperature, indicating a different sensing mechanism from a chemically functionalized SCNP sensor.

Ultra-sensitive Pressure sensor based on guided straight mechanical cracks.

Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.

Transparent ITO mechanical crack-based pressure and strain sensor

Sensors to detect motion with high precision have been extensively studied in diverse engineering research fields. Among them, transparent devices, which have strong adaptability in various fields such as display panels, have not gained much academic interest. In this study, we present a highly sensitive pressure and strain sensor based on a cracked transparent epilayer, indium-tin oxide (ITO), deposited on a transparent PET substrate. This sensor system, with which we demonstrate how to detect pressure and finger motions, exhibits ultra-sensitivity to strain (gauge factor about 4000 at 2% strain), pressure (sensitivity is about 1.91 kPa−1 at pressures from 30 to 70 kPa), and transparency (up to 89% at a wavelength of 560 nm). Also, durability has been validated over 5000 cycles. The sensor thus boasts broad applications including touchscreens and motion detectors.

Fragmentation of Fe2O3 nanoparticles driven by a phase transition in a flame and their magnetic properties

The size and crystalline phase changes of Fe2O3 nanoparticles formed in a H2/O2 flame have been investigated. At flame temperatures below 1350 °C, the mean particle size increased monotonously with the distance from the burner edge; but in high-temperature flames above 1650 °C, it suddenly decreased from 20 to ∼3 nm with the distance from the burner edge. The results of X-ray diffraction and high-resolution transmission electron microscopy showed that this sudden reduction of the size of nanoparticles was accompanied by a partial phase transformation from the metastable γ-Fe2O3 into α-Fe2O3. We suggest the structural instability due to γ- to α-phase transformation as a mechanism for a rapid fragmentation of 20 nm particles into 3 nm ones.